posterior triangle (of the neck)

The posterior triangle is made up
of the occipital and subclavian triangles.

The posterior triangle of the neck is the triangular space behind the sternomastoid,
a muscle that passes obliquely across the neck, from the sternum and clavicle
below, to the mastoid process and occipital bone above. The similar space
in front of this muscle is called the anterior triangle.

Behind the posterior triangle is the anterior margin of the trapezius.
The base of the triangle is formed by the middle third of the clavicle;
its apex, by the occipital bone. The space is crossed, about 2.5 centimeters. above
the clavicle, by the inferior belly of the omohyoid, which divides it into
two smaller triangles, an upper or occipital, and a lower or subclavian.

The occipital triangle – the larger division of the
posterior triangle – is bounded, in front, by the sternomastoid; behind,
by the trapezius; and below, by the omohyoid. Its floor is formed from above
downward by the splenius capitis, levator
scapulae, and the scaleni medius and posterior. It is covered by skin,
superficial and deep fasci�, and by the platysma below. The accessory
nerve is directed obliquely across the space from the sternomastoid,
which it pierces, to the under surface of the trapezius; below, the supraclavicular
nerves, transverse cervical vessels, and upper part of the brachial
plexus cross the space. A chain of lymph glands is also found running
along the posterior border of the sternomastoid, from the mastoid process
to the root of the neck.

The subclavian triangle is bounded, above, by the inferior
belly of the omohyoid; below, by the clavicle;
its base is formed by the posterior border of the sternomastoid. Its floor
is formed by the first rib with the first digitation of the serratus anterior.
The size of the subclavian triangle varies with the extent of attachment
of the clavicular portions of the sternomastoid and trapezius, and also
with the height at which the omohyoid crosses the neck. Its height also
varies according to the position of the arm, being diminished by raising
the limb, on account of the ascent of the clavicle, and increased by drawing
the arm downward, when that bone is depressed. This space is covered by
the integument, the superficial and deep fasciae, and the platysma, and
crossed by the supraclavicular nerves. Just above the level of the clavicle,
the third portion of the subclavian
artery curves laterally and downward from the lateral margin of the scalenus anterior, across the
first rib, to the axilla, and this is the
situation most commonly chosen for ligaturing the vessel. Sometimes this
vessel rises as high as 4 cm above the clavicle; occasionally, it passes
in front of the scalenus anterior, or pierces the fibers of that muscle.
The subclavian vein lies behind
the clavicle, and is not usually seen in this space; but in some cases it
rises as high as the artery, and has even been seen to pass with that vessel
behind the scalenus anterior. The brachial plexus of nerves lies above the
artery, and in close contact with it. Passing transversely behind the clavicle
are the transverse scapular vessels; and traversing its upper angle in the
same direction, the transverse
cervical artery and vein. The external
jugular vein runs vertically downward behind the posterior border of
the sternomastoid, to terminate in the subclavian vein; it receives the
transverse cervical and transverse scapular veins, which form a plexus in
front of the artery, and occasionally a small vein which crosses the clavicle
from the cephalic. The small nerve to the subclavius also crosses this triangle
about its middle, and some lymph glands are usually found in the space.